Jean Girard

The role of the lipogenic pathway in the development of hepatic steatosis

Non-alcoholic fatty liver disease (NAFLD) represents a wide spectrum of diseases, ranging from simple fatty liver (hepatic steatosis) through steatosis with inflammation and necrosis to cirrhosis. NAFLD, which is strongly associated with obesity, insulin resistance and type 2 diabetes, is now well recognized as being part of the metabolic syndrome. The metabolic pathways leading to the development of hepatic steatosis are multiple, including enhanced non-esterified fatty acid release from adipose tissue (lipolysis), increased de novo fatty acids (lipogenesis) and decreased beta-oxidation. Recently, several mouse models have helped to clarify the molecular mechanisms leading to the development of hepatic steatosis in the pathogenesis of NAFLD. This review describes the models that have provided evidence implicating lipogenesis in the development and/or prevention of hepatic steatosis.

Novel insights into ChREBP regulation and function

Glucose is an energy source that also controls the expression of key genes involved in energetic metabolism through the glucose-signaling transcription factor carbohydrate response element-binding protein (ChREBP). ChREBP has recently emerged as a central regulator of glycolysis and de novo fatty acid synthesis in liver, but new evidence shows that it plays a broader and crucial role in various processes, ranging from glucolipotoxicity to apoptosis and/or proliferation in specific cell types. However, several aspects of ChREBP activation by glucose metabolites are currently controversial, as well as the effects of activating or inhibiting ChREBP, on insulin sensitivity, which might depend on genetic, dietary or environmental factors. Thus, much remains to be elucidated. Here, we summarize our current understanding of the regulation and function of this fascinating transcription factor.

Factors Affecting the Secretion of Insulin and Glucagon by the Rat Fetus

Insulin (IRI) and glucagon (IRG) increased in the plasma of the rat fetus from 18½ to 20½ days of gestation and decreased on day 21½. The demonstrated failure of insulin and glucagon to cross the placenta of the rat allowed the conclusion that fetal rat pancreas secreted IRI and IRG in the plasma of the fetus at the end of gestation. Fetal hyperglycemia induced by perfusing pregnant rats with glucose for one hour produced a marked increase in fetal plasma IRI but did not modify plasma IRG. Fetal hypoglycemia induced by perfusing pregnant rats with insulin for one hour produced a fall in fetal plasma IRI but no rise in fetal plasma IRG. Chronic fetal hypoglycemia produced by fasting pregnant rats for ninety-six hours or by intrauterine growth retardation of fetuses increased fetal plasma IRG and decreased fetal plasma IRI. Norepinephrine injection in term rat fetus increased plasma IRG and decreased plasma IRI. Acetylcholine injection increased both plasma IRI and IRG, whereas serotonin remained without effect. These data suggest that IRI and IRG secretion by the fetal rat pancreas is controlled by both the blood glucose level and autonomic nervous system activity. Lower blood glucose levels and higher plasma IRI and IRG levels in pregnant than nonpregnant rats are also reported.

Role of ChREBP in hepatic steatosis and insulin resistance

Non‐alcoholic fatty liver disease is tightly associated with insulin resistance, type 2 diabetes and obesity, but the molecular links between hepatic fat accumulation and insulin resistance are not fully identified. Excessive accumulation of triglycerides (TG) is one the main characteristics of non‐alcoholic fatty liver disease and fatty acids utilized for the synthesis of TG in liver are available from the plasma non‐esterified fatty acid pool but also from fatty acids newly synthesized through hepatic de novo lipogenesis. Recently, the transcription factor ChREBP (carbohydrate responsive element binding protein) has emerged as a central determinant of lipid synthesis in liver through its transcriptional control of key genes of the lipogenic pathway, including fatty acid synthase and acetyl CoA carboxylase. In this mini‐review, we will focus on the importance of ChREBP in the physiopathology of hepatic steatosis and insulin resistance by discussing the physiological and metabolic consequences of ChREBP knockdown in liver of ob/ob mice.

Identification of the Rat Adapter Grb14 as an Inhibitor of Insulin Actions

We cloned by interaction with the β-subunit of the insulin receptor the rat variant of the human adapter Grb14 (rGrb14). rGrb14 is specifically expressed in rat insulin-sensitive tissues and in the brain. The binding of rGrb14 to insulin receptors is insulin-dependent in vivo in Chinese hamster ovary (CHO) cells overexpressing both proteins and importantly, in rat liver expressing physiological levels of proteins. However, rGrb14 is not a substrate of the tyrosine kinase of the receptor. In the two-hybrid system, two domains of rGrb14 can mediate the interaction with insulin receptors: the Src homology 2 (SH2) domain and a region between the PH and SH2 domains that we named PIR (forphosphorylated insulin receptor-interactingregion). In vitro interaction assays using deletion mutants of rGrb14 show that the PIR, but not the SH2 domain, is able to coprecipitate insulin receptors, suggesting that the PIR is the major binding domain of rGrb14. The interaction between rGrb14 and the insulin receptors is almost abolished by mutating tyrosine residue Tyr1150 or Tyr1151 of the receptor. The overexpression of rGrb14 in CHO-IR cells decreases insulin stimulation of both DNA and glycogen synthesis. These effects are accompanied by a decrease in insulin-stimulated tyrosine phosphorylation of IRS-1, but insulin receptor autophosphorylation is unaltered. These findings suggest that rGrb14 could be a new downstream signaling component of the insulin-mediated pathways.

Glucose 6-phosphate, rather than xylulose 5-phosphate, is required for the activation of ChREBP in response to glucose in the liver

BACKGROUND & AIMS In liver, the glucose-responsive transcription factor ChREBP plays a critical role in converting excess carbohydrates into triglycerides through de novo lipogenesis. Although the importance of ChREBP in glucose sensing and hepatic energy utilization is strongly supported, the mechanism driving its activation in response to glucose in the liver is not fully understood. Indeed, the current model of ChREBP activation, which depends on Serine 196 and Threonine 666 dephosphorylation, phosphatase 2A (PP2A) activity, and xylulose 5-phosphate (X5P) as a signaling metabolite, has been challenged. METHODS We inhibited PP2A activity in HepG2 cells through the overexpression of SV40 small t antigen and addressed the importance of ChREBP dephosphorylation on Ser-196 using a phospho-specific antibody. To identify the exact nature of the metabolite signal required for ChREBP activity in liver, we focused on the importance of G6P synthesis in liver cells, through the modulation of glucose 6-phosphate dehydrogenase (G6PDH) activity, the rate-limiting enzyme of the pentose phosphate pathway in hepatocytes, and in HepG2 cells using both adenoviral and siRNA approaches. RESULTS In contrast to the current proposed model, our study reports that PP2A activity is dispensable for ChREBP activation in response to glucose and that dephosphorylation on Ser-196 is not sufficient to promote ChREBP nuclear translocation in the absence of a rise in glucose metabolism. By deciphering the respective roles of G6P and X5P as signaling metabolites, our study reveals that G6P produced by GK, but not X5P, is essential for both ChREBP nuclear translocation and transcriptional activity in response to glucose in liver cells. CONCLUSIONS Altogether, our study, by reporting that G6P is the glucose-signaling metabolite, challenges the PP2A/X5P-dependent model currently described for ChREBP activation in response to glucose in liver.

Distinct regulation of adiponutrin/PNPLA3 gene expression by the transcription factors ChREBP and SREBP1c in mouse and human hepatocytes

BACKGROUND & AIMS The adiponutrin/PNPLA3 (patatin-like phospholipase domain-containing protein 3) variant I148M has recently emerged as an important marker of human fatty liver disease. In order to understand the role of the adiponutrin/PNPLA3 protein, we investigated the regulation of its expression in both human and mouse hepatocytes. METHODS Adiponutrin/PNPLA3 and lipogenic enzyme expression was determined by real-time PCR analysis in a wide panel of analysis in vivo in the mouse liver and in vitro in murine hepatocytes and human hepatocyte cell lines infected with ChREBP or SREBP1c-expressing adenoviruses. RESULTS We show that in the mouse liver, adiponutrin/PNPLA3 gene expression is under the direct transcriptional control of ChREBP (carbohydrate-response element-binding protein) and SREBP1c (sterol regulatory element binding protein1c) in response to glucose and insulin, respectively. In silico analysis revealed the presence of a ChoRE (carbohydrate response element) and of a SRE (sterol response element) binding site on the mouse adiponutrin/PNPLA3 gene promoter. Point mutation analysis in reporter gene assays identified the functional response of these two binding sites in the mouse adiponutrin/PNPLA3 promoter. In contrast, in human immortalized hepatocytes and in HepG2 hepatoma cells, only SREBP1c was able to induce adiponutrin/PNPLA3 expression, whereas ChREBP was unable to modulate its expression. CONCLUSIONS All together, our results suggest that adiponutrin/PNPLA3 is regulated by two key factors of the glycolytic and lipogenic pathways, raising the question of its implication in the metabolism of carbohydrates and lipids.

In vivo insulin resistance during pregnancy in the rat

SummaryThe glucose disappearance rate measured after IV glucose injection (1 g/kg body wt) remained unchanged between 12 and 21 day of gestation in the rat. In contrast, insulin secretion in response to IV glucose was markedly increased on day 19 and 21 of pregnancy, suggesting resistance to endogenous insulin. Glucose kinetics (glucose production, utilization and clearance) in response to various doses of IV insulin have been studied in anaesthetised postabsorbtive 19 day pregnant and virgin rats using 6-3H glucose. With the supramaximal dose of insulin (4 U/kg body wt) no differences in glucose kinetics were found between pregnant and virgin rats. In contrast, with the two lower doses of insulin (0.15 and 0.05 U/kg body wt) glucose production was inhibited by 36±3% and 13±2% (Mean±SEM) respectively in virgin rats, but was not decreased in pregnant rats. When the effect of insulin on glucose clearance was expressed as % of the maximal effect obtained with 4 U/kg body weight, the rise in glucose clearance in response to the two lower doses of insulin (0.15 and 0.05 U/kg body wt) was lower in pregnant (57.5±6 and 27.4±4%) than in virgin rats (73.3±6 and 42.2±7%). These results suggest that a decreased sensitivity to insulin appears in late pregnancy in the rat and could involve both liver and skeletal muscle.

A novel cytosolic dual specificity phosphatase, interacting with glucokinase, increases glucose phosphorylation rate.

A novel protein was cloned from a rat liver cDNA library by interaction with the liver glucokinase. This protein contained 339 residues and possessed a canonical consensus sequence for a dual specificity phosphatase. The recombinant protein was able to dephosphorylate phosphotyrosyl and phosphoseryl/threonyl substrates. We called this protein the glucokinase-associated phosphatase (GKAP). The GKAP partially dephosphorylated the recombinant glucokinase previously phosphorylated,in vitro, by protein kinase A. The GKAP fused with green fluorescent protein was located in the cytosol, where glucokinase phosphorylates glucose, and not in the nucleus where the glucokinase is retained inactive by the glucokinase regulatory protein. More importantly, the GKAP accelerated the glucokinase activity in a dose-dependent manner and with a stoichiometry compatible with a physiological mechanism. This strongly suggested that the interaction between GKAP and glucokinase had a functional significance. The cloning of this novel protein with a dual specificity phosphatase activity allows the description of a possible new regulatory step in controlling the glycolysis flux.

Enhancing liver mitochondrial fatty acid oxidation capacity in obese mice improves insulin sensitivity independently of hepatic steatosis.

BACKGROUND & AIMS Despite major public health concern, therapy for non-alcoholic fatty liver, the liver manifestation of the metabolic syndrome often associated with insulin resistance (IR), remains elusive. Strategies aiming to decrease liver lipogenesis effectively corrected hepatic steatosis and IR in obese animals. However, they also indirectly increased mitochondrial long-chain fatty acid oxidation (mFAO) by decreasing malonyl-CoA, a lipogenic intermediate, which is the allosteric inhibitor of carnitine palmitoyltransferase 1 (CPT1A), the key enzyme of mFAO. We thus addressed whether enhancing hepatic mFAO capacity, through a direct modulation of liver CPT1A/malonyl-CoA partnership, can reverse an already established hepatic steatosis and IR in obese mice. METHODS Adenovirus-mediated liver expression of a malonyl-CoA-insensitive CPT1A (CPT1mt) in high-fat/high-sucrose (HF/HS) diet-induced or genetically (ob/ob) obese mice was followed by metabolic and physiological investigations. RESULTS In association with increased hepatic mFAO capacity, liver CPT1mt expression improved glucose tolerance and insulin response to a glucose load in HF/HS and ob/ob mice, showing increased insulin sensitivity, and corrected IR in ob/ob mice. Surprisingly, hepatic steatosis was not affected in CPT1mt-expressing obese mice, indicating a clear dissociation between hepatic steatosis and IR. Moreover, liver CPT1mt expression rescued HF/HS-induced impaired hepatic insulin signaling at the level of IRS-1, IRS-2, Akt, and GSK-3β, most likely through the observed decrease in the HF/HS-induced accumulation of lipotoxic lipids, oxidative stress, and JNK activation. CONCLUSIONS Enhancing hepatic mFAO capacity is sufficient to reverse a state of IR and glucose intolerance in obese mice independently of hepatic steatosis.